| William S. Day | |
| Office: | 32nd Speaker of the Washington House of Representatives |
| Term Start: | January 14, 1963 |
| Term End: | January 11, 1965 |
| Predecessor: | John L. O'Brien |
| Successor: | Robert M. Schaefer |
| State Senate1: | Washington |
| District1: | 4th |
| Predecessor1: | Karl V. Herrmann |
| Successor1: | Bob McCaslin Sr. |
| State House2: | Washington |
| District2: | 4th |
| Predecessor2: | Mrs. John W. Epton (Kathryn) |
| Successor2: | James P. Kuehnle |
| Birth Name: | William Scott Day |
| Birth Date: | 31 January 1923 |
| Birth Place: | Rockford, Illinois, U.S. |
| Death Place: | Spokane, Washington, U.S. |
| Party: | Democratic |
William Scott Day (January 31, 1923 - May 27, 1984) was an American politician in the state of Washington. He served in the Washington House of Representatives from 1959 to 1969 and in the Senate from 1969 to 1981.[1] He was Speaker of the House from 1963 to 1965.[2]